Cryptosporidium

Cryptosporidium refers to a group of microorganisms of a variety of species which cause a human disease called cryptosporidiosis. Cryptosporidium parvum, often called "Crypto," is the most common.

Cryptosporidium is a single-celled protozoan and parasite that lives in human or animal intestines. It is excreted in feces in the form of a dormant but hardy, thick-walled oocyst (“O-o-sist”), or fertilized egg. When ingested, it emerges from the oocyst and infects the intestine lining. Size of the Oocyst is 3 to 5 microns in diameter.

Although it is not eliminated by typical disinfectant chemicals (such as chlorine bleach), it is killed at temperatures above 160°F (71°C) or by desiccation (extreme drying).

Crypto is found in every region of the planet in soil, food, and water.

It was rare until the 1980s. The largest outbreak of the ailment was in 1993 in Milwaukee, where 400,000 became ill and as many as 100 died.

According to Water Technology Magazine:

Symptoms: Diarrhea, abdominal cramps, nausea, low-grade fever, dehydration, weight loss. Symptoms appear within two to 10 days after infection, rarely last more than two weeks.

Effects: No symptoms for some. Others with healthy immune systems have symptoms for one or two weeks. Symptoms can recur. Severe illness in those with weaker immune systems.

It is regulated by the EPA with a Maximum Contaminant Level (MCL) in drinking water of zero.

Treatment: Crypto can be reduced or removed from water by carbon adsorption, reverse osmosis, nanofiltration, ultrafiltration--by any filtration of absolute 1 micron rating. It is also controlled by ozonation and distillation. Chlorination is not a recommended treatment.

NSF/ANSI has treatment standards under Standards 53 (Drinking Water Treatment Units), 55 (Ultraviolet), and 58 (Reverse Osmosis).
In many situations, ultraviolet has become the preferred treatment.

Sources: US Environmental Protection Agency (EPA), US Centers for Disease Control and Prevention, Water Quality Association, NSF International, US Department of Agriculture.Water Technology Magazine,Volume 31, Issue 3 - March 2008